The following excerpt is adapted from a segment about the fungal disease valley fever from the April 24, 2020 episode of the WNYC Studios show Science Friday. Read the rest of the piece here, and hear the full Science Friday segment including an interview with FM89's Kerry Klein here.
The life of the Valley Fever fungus, and the root of the problem, begins beneath our feet. There are currently two known species of Coccidioides, and both cause disease: Coccidioides immitis, is found in California and in Washington state, and Coccidioides posadasii, which is found all over the western part of North America and parts of Central and South America. Both Cocci species tend to grow in the top layers of the earth, and like alkaline, salty soil—like the hard packed clay and sandy loams that makes up much of the American Southwest.
“The [Valley Fever] hot spots are in what we call the former Bakersfield deserts in the southwest,” says Antje Lauer, a microbial ecologist at California State University, Bakersfield.
Little organic matter typically exists in this soil. It’s a hostile environment for many microorganisms to live in, but this is where Coccidioides thrives. The fungus could grow in more organically rich soils if given the chance, says Lauer, but these environments are dominated by other microbes that often outcompete Cocci.
“It's a little wimp,” Lauer says, and adds, “That's often the case with a lot of dangerous pathogens.”
Cocci may become a bursting bulbous sphere in the human body, but it looks drastically different in the soil. During a rainy winter or monsoon season, the fungus will begin to grow in long, filamentous strands, called mycelia.
“I think it looks like fuzzy brie,” says Anh Diep, a research graduate student in immunologist Katrina Hoyer’s lab at UC Merced, where the team keeps a mini-fridge-sized incubator filled with the fungus. Stacks of petri dishes are completely covered in puffy, cottony domes—like the fluffy stuffing that you’d pull out of a plush toy.
Although researchers have been studying how Coccidioides grows in its environment since the 1930s, scientists still don’t understand why it seems to like particular patches of dirt. “I can literally go to [one] spot year after year after year, and always find the fungus there, but I go two meters away and I don’t find it,” says Bridget Barker, a fungal ecologist and associate professor at Northern Arizona University in Flagstaff. “What is it that’s driving that really disjunct distribution in the environment?”
In desert regions, Barker pays attention to the plants and animals—any common factors that could influence the fungus. Recently, she found one. Barker and her team noticed that they kept getting positive tests for Coccidioides in soil samples taken near burrows of desert rodents, like pocket mice and kangaroo rats. These burrows might be creating perfect homes for the fungus, Barker says.
“These nice, little microenvironments have higher humidity, lower temperature so the fungus is maybe a little happier,” she says, and “the hair, the skin, the feces, the urine—whatever they’re leaving behind in the burrow environments is a good food source.”
The rodents could also potentially serve as a carrier for the disease—getting infected, dying, and decaying in the soil provides more nutrients for the fungus to feed on, she says. Now, she is teaming up with wildlife biologists at Arizona’s Game and Fish Department to collect blood samples from rodents to see if they are infected with or have been exposed to Valley Fever.
Barker is not the first to make this observation. Researchers at UC Berkeley have also found similar associations in California. Finding out what is driving the growth of the fungus in these burrows could inform people’s decisions about what plots of soil to leave alone. This could help lower the chances of releasing the fungus into the air.
“If we can understand certain times of the year, or certain areas that people are going to be more at risk for exposure than I think we can also tackle the disease in that way as well,” Barker says.
